Plasma sprayed mullite coatings on silicon-base ceramics

ABSTRACT

A silicon-base ceramic substrate is provided with a mullite chemical barrier/thermal barrier coating uniformly bonded to a surface. The coating is substantially free of amorphous mullite.

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the U.S.Government together with a subcontractor of the National ResearchCouncil and is subject to the provisions of Section 305 of the NationalAeronautics & Space Act of 1958, Public Law 85-568 (72 Stat. 435, 42 USC2457).

STATEMENT OF COPENDENCY

This application is a division of application Ser. No. 08/031,444 whichwas filed Mar. 15, 1993, now U.S. Pat. No. 5,391,404.

TECHNICAL FIELD

This invention is concerned with plasma sprayed coatings on ceramicmaterials. The invention is particularly directed to plasma sprayedmullite coatings on silicon-base ceramics.

Silicon-base ceramics have been proposed for high temperature structuralapplications, such as heat exchangers, gas turbines, and advancedinternal combustion engines. Silicon-base composites have been proposedas materials for applications in combustors for next generationsupersonic commercial airplanes. Because these materials exhibit pooroxidation resistance in reducing atmospheres and in environmentscontaining salts, water vapor or hydrogen, it is necessary to applychemical barrier coatings (CBCs) which provide protection fromchemical/environmental attack at elevated temperatures. The suggestedapplication of these materials in aircraft engines also necessitates thedevelopment of thermal barrier coatings (TBCs) which are generally usedto extend the life of components exposed at high elevated temperatures.

Mullite has been proposed as a material for chemical barrier coatings aswell as thermal barrier coatings on silicon-base ceramics. Mulliteexhibits excellent environmental durability, low thermal conductivityand low coefficients of thermal expansion comparable to silicon-baseceramic materials.

In the prior art attempts were made to apply mullite coatings onsilicon-base ceramic materials because of their high potential as hightemperature structural materials. However, mullite coatings applied bytraditional plasma spray processes crack extensively and debond onthermal cycling.

It has been found that mullite coatings applied by traditional plasmaspray processes contain a large amount of amorphous mullite. Thisformation of amorphous mullite is due to the rapid solidification ofplasma sprayed molten mullite on cold substrates. The amorphous mullitethen crystallizes at about 1000° C. during thermal cycling. Theshrinkage of the mullite coating during the crystallization of amorphousmullite is the key factor causing the observed cracking and debonding onthermal cycling.

To be utilized as chemical barrier coatings or thermal barrier coatings,it is essential that mullite coatings adhere to the substrate. It isfurther critical that these materials do not crack on thermal cycling.Because of the extensive cracking and debonding, chemical/environmentalprotection and thermal barrier effects rendered by mullite coatingsprepared by traditional plasma processes have been minimal.

It is, therefore, an object of the present invention to provide adherentand durable plasma sprayed mullite coatings on silicon-base ceramics.

A further object of the invention is to provide mullite coatings thatadhere to a silicon-base ceramic substrate and do not crack on thermalcycling.

BACKGROUND ART

Dittrich U.S. Pat. No. 3,617,358 relates to the composition of mullitepowders used in flame spray coating of metals and ceramics. Watson et alU.S. Pat. No. 4,039,480 describes mullite coated spherical balls used inhigh temperature automotive applications, such as catalytic converters.Heating is disclosed for the purpose of sintering after the applicationof the coating.

Jones et al U.S. Pat. No. 4,963,436 is directed to the production ofmullite hollow spheres used as catalysts or in packing columns. Hereagain, heating is utilized for sintering after the application of thecoating.

Interrante U.S. Pat. No. 4,060,654 is concerned with a method ofapplying aluminosilicate coatings, such as mullite, which permits theuse of known high temperature oxides as coating materials. Heating isrelied on after the application of the amorphous mullite.

Williams U.S. Pat. No. 4,528,038 is concerned with the use of mullitecoatings in which the coating is heated and components dipped into abath for coating the substrate. Heating follows the coating process.Williams U.S. Pat. No. 4,554,186 relates to heating the substratematerial after the application of the mullite coating.

DISCLOSURE OF THE INVENTION

According to the present invention mullite coatings are plasma sprayedonto silicon-base ceramic materials. More particularly, a crystallinemullite powder is heated and flame-sprayed onto a preheated substrate.

The substrate is heated to a temperature in excess of 800° C. during thespraying process. At such temperatures mullite will immediatelycrystallize as it solidifies.

Such high temperatures insure a slow solidification process. This slowsolidification facilitates the formation of a crystalline mullitecoating which is more immune to cracking and delamination.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, advantages, and novel features of the invention will bemore fully apparent from the following detailed description when read inconnection with the accompanying drawings wherein:

FIG. 1 is a photomicrograph at a 200 magnification showing a mullitecoating that has been plasma sprayed onto a silicon carbide substrateafter it has been heated to about 1050° C.;

FIG. 2 is a photomicrograph at a 200 magnification of the coating shownin FIG. 1 after completing two 24-hour thermal cycles at 1000° C.; and

FIG. 3 is a photomicrograph at a 200 magnification of a mullite coatingthat has been plasma sprayed onto a silicon carbide substrate after ithas been heated to about 1250° C. and after completing ten 20-hourthermal cycles at 1400° C.

BEST MODE FOR CARRYING OUT THE INVENTION

Shrinkage of the coating with the resultant cracking and debonding iseliminated with a mullite coating that has been applied in accordancewith the present invention. This is accomplished by preventing thedeposition of amorphous mullite by heating the substrate to a very hightemperature during the spraying process. The substrate is heated to atemperature greater than about 800° C. during spraying. In the preferredembodiment, the substrate is heated to a temperature greater than about1000° C. which is the crystallization temperature of amorphous mullite.It is further contemplated that the substrate be heated to a temperaturegreater than about 1200° C.

The beneficial technical effects of the present invention areillustrated by silicon carbide substrates coated in accordance with thepresent invention. The silicon carbide substrates were 2.54 centimetersby 0.45 centimeters by 0.2 centimeters coupons. The coupons are acommercially available alloy, known as Hexoloy, which is a registeredtrademark of the Carborundum Company of Niagara Falls, N.Y.

These coupons were etched in molten sodium carbonate. The etching wasperformed by heating to about 860° C. to 1000° C. for about 8-10 hoursto produce a rough surface on each coupon.

The etched coupons were then oxidized at 1000° C. in air for about 28 toabout 30 minutes to burn out residual carbon on the etched surface. Thecoupons were subsequently immersed in 10% HF for 24 hours to dissolvesilica.

The spray powder was mullite which was purchased from a commercialvendor with a powder size of -62/+20 μm. The coating parameters were:

    ______________________________________    Plasma gun power      36-53 kw    Plasma arc gas        Ar-40% He    Ar powder carrier     2-6 SLPM    gas flow rate    ______________________________________

The substrate was placed inside a furnace while the temperature wasmonitored with a thermocouple. Mullite powder was fed when apredetermined temperature listed above was reached.

Post test metallographic examination of the coatings applied atsubstrate temperatures of about 800° C. and about 1300° C. reveal thecoatings were well bonded to the substrate. As shown in FIG. 1, nocracks were observed.

The coatings were thermally cycled between room temperature and up toabout 1400° C. For each cycle, the coupon was inserted to a preheatedfurnace in air, held in the furnace for about 20 to 24 hours, and airquenched to room temperature. Metallographic examination of thethermally-cycled coatings revealed that the coatings were still wellbonded and only a few microcracks were observed as illustrated in FIGS.2 and 3. The observed microcracks were not interconnected. Thus, thesemicrocracks would not act as paths for oxidants.

It is apparent from FIGS. 1, 2, and 3 as well as the above explanation,that the coating process can be used to prepare adherent and durablemullite coatings on silicon-base ceramic material. These may be chemicalbarrier coatings or thermal barrier coatings. Because of the excellentadherence and coherence, chemical/environmental protection and thermalbarrier effects rendered by the mullite coatings prepared by thisprocess are superior to mullite coatings prepared by traditional plasmaspray processes.

The extensive cracking and debonding of conventionally prepared mullitecoatings which contain a large amount of amorphous mullite have beeneliminated. This results from the mostly crystalline mullite coatingsobtained by heating the substrate to a very high temperature which isgreater than about 800° C., preferably greater than about 1000° C., andoptimally greater than about 1200° C. during the coating process.

While the preferred embodiment of the invention has been disclosed anddescribed, it will be appreciated that the various proceduralmodifications may be made to the process without departing from thespirit thereof or the scope of the subjoined claims. By way of example,it is contemplated that other silicon-base ceramics, such as Si₃ N₄ orcomposites based on SiC or Si₃ N₄, or other ceramics, metals, or metalalloys may be used for the substrate. The plasma spray parameters,substrate temperature, and substrate shape may be modified from thosedescribed above. Also, the substrate may be heated by other methods,such as a heating lamp or resistance heating.

What is claimed is:
 1. A coated article of manufacture consisting ofasilicon-base ceramic substrate, and a substantially crack free mullitecoating uniformly bonded to a surface of said substrate, said coatingbeing substantially crystalline and free of amorphous mullite.
 2. Acoated article as claimed in claim 1 wherein the substrate is siliconcarbide.
 3. A coated article as claimed in claim 1 wherein the substrateis a SiC/SiC composite.
 4. A coated article as claimed in claim 1wherein the substrate is Si₃ N₄.
 5. A coated article as claimed in claim1 wherein the substrate is a Si₃ N₄ /Si₃ N₄ composite.
 6. A coatedarticle of manufacture for high temperature structural applicationsconsisting ofa substrate of silicon carbide, and a substantially crackfree chemical barrier/thermal barrier coating of mullite uniformlybonded to a surface of said substrate, said coating being substantiallyfree of amorphous mullite.
 7. A coated article as claimed in claim 6wherein the substrate is a SiC/SiC composite.
 8. A coated article asclaimed in claim 6 wherein the substrate is Si₃ N₄.
 9. A coated articleas claimed in claim 8 wherein the substrate is a Si₃ N₄ /Si₃ N₄composite.